Other than bad optic clarity, what's wrong with buying the cheaper stuff? I'm seeing a lot of people talkin about good <$100 dollar red dots and such - is there a general reason why it's a bad idea?

In terms of weaponry, it'd be on an ar-15 - I imagine that other weapons would have enough recoil to shatter the cheap lens?

I highly doubt that the lense will shatter.  I've shot a 300 mag with a cheap scope and nothing shattered.  But the sight picture would have been much better with a quality scope.

"People", you mean folks that are new to optics  Cheap scopes suck, many of them have a yellow tint and the resolution is horrible.  They also don't stand up to recoil and the minor rigors of range use, never mind water submersion, survival from a 5ft drop, etc.

They are cheap for good reasons.  If you buy them you will very likely throw it away soon enough, they're a waste of money.

I imagine losing zero, lack of confidence in your optics, crappy build quality and poor optical quality would probably put most people off. I know for a fact that i will never put optics on a weapon that i am not 100 percent confident in.

Usual failures revolve around inconsistant adjustments, settings of, or even the entire illumination not working and needing to be whacked to turn back on, cheap switches that break, non waterproofed, poor opticla quality, excessive dark tint for dots, general unreliability, battery life, etc.

For 2nd focal plane vaiables you get POI shift with shifting magnification, for magnified optics in general crap inside the optics like dark spots, and of course lts of the thing s listed in paragraph one.

If none of that bothers you a cheap optic is right up your alley.  For example... remember not to use setting 5 becasue its broken, if it flickers whack it, the on/off switch is broken so use the brightness adjustment turned to minimum to increase battery life when optic should be off, or not minding some of the LEDs that make up your reticle have died from less than .223 recoil and so on... I have experienced all these.

Have fun.

To be honest losing zero if you dont do any adjusting is the rareest of all cheap optic problems but it does happen... I have never seen it.

The biggest problem that I have seen with other people's poor quality optics has not so much to do with optical clarity; it is the inability to hold zero or adjustment.....NcStar being the classic example.  

Then there is also the reliability consideration; things tend to go TU at the worst possible time.

If you are comfortable with your projectile not hitting where you aim on a consistent basis, or never knowing whether your optic will work when you need it, then by all means, get the cheap crap.

This is not to suggest that "value" optics are not available with reasonable compromise as compared to higher quality stuff, but sub $100 is just asking for frustration.

I've only had 1 cheap optic that held zero. I could have bought a top of the line Luepold for what I spent on cheap optics and ammo trying to sight them in. Don't buy cheap glass.

Yep, just don't try to tell this dude

Quoted: Yep, just don't try to tell this dude

I really like the rat poison part...I bet that idiot is lapping it up like a kitten does milk.  

To the OP....optics are the last thing that one should skimp on.

He just posted pics of his rifle, what an ungainly Tapco-f*cked piece, I never did realize how ugly an AR15 could become.  Cheap crap everywhere.

He sounded like a know-it-all, now having seen his abomination of an AR15, it's confirmed

I had a revelation today: the high quality manufacturers, be it firearms, optics, hand guards, accessories, tend to produce things that are pleasing to the eye, in addition to being tough and in spec.

The cheap stuff invariably looks cheap, which makes sense I guess, if they skim on quality, they probably wouldn't care how it looks either.

ive had several bad experiences with fake/knockoff optics.

i was shooting my friends savage .308 when his bsa scope took a dive.  i still dont know what happened but the turrets no longer functioned and obviously the scope wasnt even close to sighted in anymore.  

i have had a couple fakepoints for my airsoft guns.  ive mounted a couple to my ar's.  most recently a comp m4.  it held it zero pretty well but it eats batteries and the mount sucks.  good enough for a toy but when batteries mysteriously die even when the optic is off, it doesnt belong on a real gun.

ymmv.  

Then you can buy $400 Eo-Techs that drain batteries and cut off by themselves and the battery lever pops up while shooting

The Aimpoint's seem to me to be overall the better constructed red dot and you can get a good used one for about $300

As for the standard "tacticool" AR 1-4 style scopes get ready to fork out some money for the big names.
You might want to try a Bushnell Elite 4200 1.25x4 in 30mm and illuminated German #4 reticle.  I put one on my Sig 556 and it is excellent for $350.